Field of the Invention
This invention relates to paging systems and more particularly to a paging receiver for receiving information having analog voice messages, digitizing the analog voice messages, and storing the voice messages in a memory for playback.
Communication systems in general and paging systems in particular using transmitting call signals have attained widespread use for calling selected receivers to transmit information from a base station transmitter to the receivers. Modern paging systems and paging receivers in particular have achieved multifunction capabilities through the use of microcomputers which allow the paging receiver to respond to information having various combinations of tone, tone and voice, or data messages. This information has been transmitted using a number of well known paging coding schemes and message formats.
In the operation of such paging receivers, important factors involved in their operation have been the portability of the receiver, the size of the paging receiver, the cost of manufacturing the paging receiver, the limited energy available for the paging receiver, the limited availability of the radio spectrum, the fast response time required in today's active society, and the number of paging receivers included in the paging system. In such paging receivers, in order that the drain on the battery may be minimized, the paging receiver has been systematically turned off and turned on to maximize the length of time energy is available from the battery (battery saving). The limited energy in which the paging receiver must operate constrains the type of electronic circuitry available for such paging receivers.
Prior voice type paging systems have used analog voice channels for the transmission and reception of voice messages. While certain types of paging systems have used binary signalling formats, transmission in an analog form has remained the most common technique for voice signalling. Prior analog paging receivers that have used analog representation to store voice signals have been limited in several features. These include the ability to store voice messages in a reasonable size memory to recall at a later time, use of digital modulation techniques to store and reconstruct voice messages, and the ability to increase throughput in a paging RF channel receiver. Digital processing of voice messages is, in general, qualitatively superior to analog processing. This has been a result of the fact that once voice messages are in a digitally represented form, they are not subject to the type of signal degradation that usually occurs in analog processing. Thus, it is beneficial to represent the voice message in digital form rather than analog form being subject to the type of distortion inherent in analog processing techniques.
Another problem with prior analog voice paging receivers has been in the ability to store a plurality of voice messages and selectively recall a particular voice message. One prior analog voice paging receiver attempted to overcome this problem by storing the voice information on conventional analog magnetic tape (U.S. Pat. No. 4,356,519). While such a voice type paging receiver has been available, it has been commercially unfeasible. Some of the reasons has been the high cost of the electronic components, the low battery life from the high drain of current required by the tape mechanism, and the difficulty in operating the pager in a battery saving environment. Additionally, if a sequence of messages have been stored on a tape, the ability to recall a single message has been hampered by the inability of the analog magnetic tape to randomly select the selected message. Thus, prior analog voice paging systems were developed without the ability of the paging receiver to store voice messages.
A further problem with analog voice paging systems has been the transmission of a two-second alert gap before each voice message as illustrated in the paging format of FIG. 1. The alert time gap has been a necessity since prior analog voice paging receivers could not store messages and needed to generate an alert before the voice message to notify the paging receiver user of an incoming voice message. In the prior analog voice paging receivers, when the alert on the analog voice paging receiver sounded, the user had to be present to receive the message. As is evident, with a paging receiver capable of storing voice messages, the user does not have to be present to receive the message. Eliminating the gap would be extremely beneficial because message transmission throughput on the RF channel would increase.
Turning now to a disadvantage that has arisen with respect to digital stored voice paging receivers having fixed length allocated storage areas to store the voice message. Applicants have discovered a problem resulting from the recording of channel noise in the storage area after the reception of a voice message. This condition occurs when the length (transmission time) of the voice message is less than the storage capacity of the storage area. In this case, when the voice message is less than the capacity of the storage area, channel noise has been stored in the storage area until the storage area has been filled. This channel noise may take the form of white noise or the next message transmitted on the RF channel. During playback, this has resulted in the user hearing the voice message followed immediately by undesirable channel noise or the remainder of a previously stored message if the storage of the current message is aborted by the receipt of a new page (i.e. back-to-back pages). As is evident, it would be very desirable to eliminate this annoying problem.